This is a 12 dB per octave add-on scratch and rumble fill-r which can be connected into the ‘tape Monitor’ or some similar facility of the amplifier.
It is a conventional second order filter circuit having passive high pass filter formed by the series capacitance C3 and C4, plus the parallel resistance of R2 and R3 (the latter also being used to bias emitter follower transistor Q1). A passivefilter of this type gives only a very slow initial roll off, and an ultimate attenuation rate of only 6 dB per octave. A bootstrapping resistor is therefore used to improve performance. Above the cut-off frequency, where the gain of the circuit would otherwise fall off somewhat, R1 has the effect of reinforcing the input signal. Well below the cut off frequency, losses through C4 result in the signal level at Q1 emitter being well below that at the junction of C3 and C4. This results in some of the signal at the junction of C3 and being tapped off through R1, with C3 and R1 effectively forming a second high passfilter network. This eliminates the sluts, initial roll off rate (in fact there is a small and insignificant peak of about 0.5dB above the cut off frequency) and speeds up the attenuation rate to a nominal 12dB per octave.
The low pass filter works in much the same way as the high pass one, except of course, the R and C filter elements have been transposed so as to give the correct filter action.
With the specified component values the rumble filter response falls below unity at approximately 45Hz, reaches the -6 d13 point just above 30Hz, and then falls away at a nominal 12dB per octave. Thescratch filter response crosses the unity gain point at about 6k5Hz, reaches the -6dB point at approximately 10kHz, and then falls away at a nominal 12dB per octave.
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